cranial vertebrae. To complete the parallel between the
development of the skull and of the vertebrae, it would have been
necessary to show that the side walls of the cranium developed in a
similar manner from separate pieces. Mueller could not prove this point
from the available embryological data, and indeed the facts which he did
use had to be twisted to suit his theory. A curious apparent
confirmation of his idea that the centra of the cranial vertebrae are
formed from separate halves was supplied in 1839 by Rathke's discovery
of the trabeculae in the embryonic skull of the adder.

The next big step in the study of the development of the skull was
taken by a pupil of Mueller, C. B. Reichert, who showed in his work
very distinct traces of his master's influence. Reichert's first and
most important contribution to the subject was his paper on the
metamorphosis of the gill, or, as he called them, the visceral arches
in Vertebrates,[204] particularly in the two higher classes. Reichert
describes the similar origin in embryo of bird and mammal (pig) of
three "visceral" arches. These arches stand in close relation to the
three cranial vertebrae which Reichert, like Mueller, distinguishes. He
makes the retrograde step of admitting only three aortic arches, and
he is not inclined to consider the three visceral arches as equivalent
to the gill-arches of fish--in his opinion they have more analogy with
ribs, though differing somewhat from ribs in their later
modifications. The visceral arches are processes of the visceral
plates (von Baer), which grow downwards and meet in the middle line,
leaving between one another and the undivided body wall three visceral
slits opening into the pharynx. The first visceral process is
different in shape from the others, for it sends forward, parallel
with the head and at right angles to its downward portion, an upper
portion in which later the upper jaw is formed. The other two
processes are straight. From the hinder edge of the second visceral
arch there develops, as Rathke had seen, a fold which is comparable
with the operculum of fish. The first slit develops externally into
the ear-passage, internally into the Eustachian tube, and in the
middle a partition forms the tympanic ring and tympanum. Inside each
of the visceral processes on either side a cartilaginous rod develops.
In the first process this rod shows three segments, of which the first
lies inside that portion of the process wh